Passive drag modification system

Abstract

The present invention is directed to a micro-array surface that provides for drag reduction. An aerodynamic or hydrodynamic wall surface that is configured to modify a fluid boundary layer on the surface is provided. The wall surface has a plurality of cavities defined therein the surface. In various examples, the interaction of the cavities with a flow of fluid relative to the wall surface is configured to form a plurality of stable, embedded cavity vortices such that a partial slip condition is produced over the wall surface.

What is claimed is: 
     
         1 . An aerodynamic or hydrodynamic wall surface configured to reduce frictional drag between the wall surface and a fluid flowing relative to the wall surface, comprising:
 a plurality of roughness elements disposed on and extending therefrom the wall surface, wherein each roughness element has a front, upstream surface and an opposing rear, downstream surface, wherein the plurality of roughness elements are positioned substantially transverse to the flow of fluid across the wall surface, wherein the plurality of roughness elements are positioned to define at least one embedded cavity therebetween successive roughness elements, wherein the flow of fluid relative to the wall surface forms at least one cavity vortex rotating therein each embedded cavity, wherein at a predetermined fluid flow rate relative to the wall surface, at least a portion of the at least one cavity vortex rotates outside of the cavity, and wherein the at least one cavity vortex reduces friction between the fluid and the wall surface.   
     
     
         2 . The wall surface of  claim 1 , wherein each cavity vortex contains a predetermined volume of fluid. 
     
     
         3 . The wall surface of  claim 2 , wherein the plurality of roughness elements are configured so that the volume of fluid therein each cavity vortex is substantially constant as fluid flows relative to the wall surface. 
     
     
         4 . The wall surface of  claim 3 , wherein the at least one cavity vortex rotating therein each embedded cavity forms a fluidized bearing surface. 
     
     
         5 . The wall surface of  claim 1 , wherein at the predetermined fluid flow rate relative to the wall surface, at least a portion of the rotating cavity vortex has a vortex height greater than a depth of the respective cavity. 
     
     
         6 . The wall surface of  claim 1 , wherein the plurality of roughness elements are positioned in successive ridges of roughness elements. 
     
     
         7 . The wall surface of  claim 6 , wherein the at least embedded cavity is formed between the successive ridges of roughness elements. 
     
     
         8 . The wall surface of  claim 1 , wherein at least a portion of at least one roughness element of the plurality of roughness elements is curved. 
     
     
         9 . The wall surface of  claim 1 , wherein at least one roughness element of the plurality of roughness element extends therefrom the wall surface at an acute angle relative to the to the portion of the wall surface from which the at least one roughness element extends. 
     
     
         10 . The wall surface of  claim 9 , wherein at least one roughness element of the plurality of roughness element extends therefrom the wall surface at an angle of about 45 degrees relative to the portion of the wall surface from which the at least one roughness element extends. 
     
     
         11 . The wall surface of  claim 1 , wherein at least one roughness element of the plurality of roughness element extends substantially normal to the underlying wall surface. 
     
     
         12 . The wall surface of  claim 1 , wherein the at least one embedded cavity has a ratio of length in the direction of fluid flow to depth of at least 1:1. 
     
     
         13 . The wall surface of  claim 12 , wherein the ratio of length in the direction of fluid flow to depth is at least 2:1. 
     
     
         14 . The wall surface of  claim 1 , wherein the front, upstream surface of each roughness element has a curved, convex cross-sectional shape relative to the flow of fluid over the wall surface. 
     
     
         15 . The wall surface of  claim 14 , wherein the rear, downstream surface of each roughness element has a curved, concave cross-sectional shape relative to the flow of fluid. 
     
     
         16 . An aerodynamic or hydrodynamic wall surface configured to reduce frictional drag between the wall surface and a fluid flowing relative to the wall surface, comprising:
 a plurality of roughness elements disposed on and extending therefrom the wall surface, wherein the plurality of roughness elements are positioned relative to the flow of fluid across the wall surface such that a cavity is defined between each successive roughness element; and   means for restricting fluid from entering or leaving each cavity to alleviate the no-slip condition formed between the wall surface and the fluid flowing relative to the wall surface.   
     
     
         17 . The wall surface of  claim 16 , wherein the means for restricting fluid from entering or leaving each cavity comprises configuring the roughness elements such that a rotating vortex is formed therein each cavity, wherein the rotating vortex restricts the flow of fluid flowing relative to the wall surface from entering each cavity. 
     
     
         18 . The wall surface of  claim 17 , wherein at a predetermined fluid flow rate relative to the wall surface, at least a portion of the rotating vortex has a vortex height greater than a depth of the cavity in which the vortex is formed. 
     
     
         19 . The wall surface of  claim 18 , wherein each cavity has a ratio of cavity length in the direction of fluid flow to depth of at least 1:1. 
     
     
         20 . The wall surface of  claim 19 , wherein the ratio of length in the direction of fluid flow to depth is at least 2:1. 
     
     
         21 . A method for reducing frictional drag between a wall surface and a fluid flowing relative to the wall surface, comprising:
 providing a plurality of roughness elements disposed on and extending therefrom the wall surface, wherein the plurality of roughness elements are positioned relative to the flow of fluid across the wall surface such that a cavity is defined between each successive roughness element; and   forming a trapped rotational vortex in each cavity.   
     
     
         22 . The method of  claim 21 , wherein at a predetermined flow rate of the fluid relative to the wall surface, a portion of the trapped rotational vortex extends out of the cavity.